The main super PAC supporting Hillary Clinton is struggling in its early efforts to line up cash toward a fundraising goal of as much as $500 million, according to sources with knowledge of its fundraising.

The group, Priorities USA Action, is trying to secure 30 or more pledges of at least $1 million apiece to be unveiled publicly when the former secretary of state officially enters the race, sources say. But, so far, it has received only about 10 firm commitments, and it is encountering resistance from top donors who have given to other Clinton-linked vehicles, including ostensibly aligned super PACs and even the apolitical Clinton Foundation.

Priorities occasionally has clashed behind the scenes with other groups working toward the same goal. The tensions spilled out into public view Monday, when Clinton ally David Brock quit the Priorities board amid accusations it was trying to boost its own fundraising by undercutting that of two pro-Clinton groups he runs.

While Brock said late Monday he was “open to returning” after Priorities pledged to address his concerns, it’s Priorities itself that some Clintonites are concerned about.

“People are starting to worry that Priorities could be a weak link,” said a strategist who has worked with the various pro-Clinton outside groups.

The stumbles — and the slow fundraising start — raise doubts about a key assumption about Clinton’s strength as a presidential candidate: that her allies will be able to rely on a seemingly bottomless well of support from rich supporters to build an historic political money juggernaut that will overwhelm prospective rivals.